In recent years, with increasing transmission traffic, demands for an improvement in transmission capacity of an optical transmission system are growing. In addition, for a short-distance transmission system, there are demands for not only an improvement in transmission capacity but also low-cost and simple configurations.
Thus, an optical transmission system applied a discrete multi-tone (DMT) modulation scheme (or format) is under consideration. The DMT modulation scheme is one example of a multi-carrier transmission technology based on an orthogonal frequency division multiplexing (OFDM) technology. For example, the DMT modulation scheme is used in the DSL technology such as an asymmetric digital subscriber Line (ADSL).
The DMT modulation scheme (may also be referred to as a “multi-carrier modulation scheme”) uses a technique called a “bit loading”. The “bit loading” allocates bits to each of subcarriers dependent on transmission characteristics of the subcarriers. The “transmission characteristics” may also be referred to as “reception characteristics” or “signal quality” or “transmission conditions”.
Exemplary indicators or indices of the transmission characteristics may include a signal-to-noise ratio (SNR) and a bit error rate (BER). The DMT scheme allocates to a subcarrier with high transmission characteristics bits more than that of a subcarrier with low transmission characteristics. Therefore, it is possible to improve a usage efficiency of frequency in a transmission band and to increase a transmission capacity.
In a case where the DMT modulation scheme is applied to an optical transmission system, an electro-optic (E/O) converter with a direct modulation scheme by using a semiconductor laser can be applied to an optical transmitter. Meanwhile, an optic-electro (O/E) converter with a light reception element such as a photo detector or photo diode (PD) can be applied to an optical receiver.
The semiconductor laser and the PD are general optical devices and so are inexpensive. Further, in the direct modulation scheme, a modulated signal is generated by modulating a driving current of the semiconductor laser that is a light source in accordance with transmission data. Upon comparing with an external modulation scheme using an optical modulator in addition to a light source, the direct modulation scheme may be difficult to enhance a modulation speed. However, since the direct modulation scheme is unnecessary to use phase information of a signal light, it is possible to decrease in sizes and costs of an optical transmission system.
Therefore, by applying the DMT modulation scheme to an optical transmission system, it is possible to provide an optical transmission system which is available to improve the frequency usage efficiency (in other words, transmission capacity) and to decrease in sizes and costs.